Wavelength division multiplexing (WDM) technology is available in the optical transmission field. Some WDM systems use an optical add-drop multiplexer (OADM) in which a wavelength selectable switch (WSS) is used. The WSS can add an optical signal with a selected wavelength into an optical signal wavelength-division-multiplexed, drop an optical signal with a selected wavelength from an optical signal wavelength-division-multiplexed. Alternatively, the WSS can switch a path of an optical signal with a selected wavelength.
In a WDM system, an optical signal in which optical signals with a plurality of different wavelengths are multiplexed is sent from an optical transmission apparatus in transmitter node. The optical signal sent from the optical transmission apparatus in transmitter node passes through an optical transmission line such as an optical fiber, and is received by the an optical transmission apparatus in receiver node, where the optical signal is split into optical signals with the different wavelength and is received for each wavelength. At least one optical transmission apparatus in repeater node may be provided between the transmitter node and the receiver node. The optical transmission apparatus in repeater node amplifies the optical signal that it has received by using, for example, an optical preamplifier or optical post-amplifier, and sends the amplified optical signal to the optical transmission apparatus in receiver node. The optical transmission apparatus in repeater node may use the above-mentioned OADM to extract (drop) an optical signal with a particular wavelength from an optical signal in which optical signals with different wavelengths are multiplexed or to include (add) an optical signal with a particular wavelength into an optical signal in which optical signals with different wavelengths are multiplexed.
During an operation in an optical communication system, a wavelength that has not been used at the start of the operation may be used. In this case, since the number of wavelengths used is increased, the level diagram may be changed. In prior art, to prevent the level diagram from being changed, dummy light is inserted for the wavelength that has not been originally used.
To insert dummy light, some light sources output light with a predetermined wavelength from light emitted from a wide-band light source, such as an amplified spontaneous emission (ASE) light source, by using a configuration formed with an arrayed waveguide grating (AWG), a combination of an optical coupler and a narrow band pass filter, a combination of an optical coupler, a circulator and a fiber grating, or a combination of a circulator, a plurality of fiber gratings and an AWG. Exemplary ASE light sources include the ASE-FL7010 manufactured by FiberLabs Inc. (searched for on the Internet on Aug. 26, 2010, URL<http://www.fiberlabs.co.jp/lightsource/ase_cband.htm>).
The number of channels of multiplexed signals may rapidly change because of, for example, a failure in an optical transmission apparatus. To cope with this problem, some optical amplifiers prevent excessive output of signal light with the remaining wavelengths escaping the failure. For example, one of these optical amplifiers amplifies part of a light signal including a light signal and dummy light included in the light signal with an optical amplifying unit, removes the dummy light from the amplified light signal and dummy light with a filter, and controls the dummy light so that the light output from the light source of the dummy light becomes to a predetermined light output with a dummy light control circuit. Japanese Laid-open Patent Publication Nos. 2000-286492 and 2002-198599 are examples of related art.